Organic semiconductor devices and organic electroluminescent devices produced by using wet process

ABSTRACT

The present invention provides a method for manufacturing an organic semiconductor thin film by a wet process with a composite solution prepared by dissolving at least two organic semiconductor compounds in a mixed organic solvent including at least two organic solvents having different volatility and having different solubilities of the organic compounds at room temperature. Due to the differences in the evaporation speeds of the solvents and the solubilities of the organic compounds, the organic compounds are continuously deposited according to the composition of solvent that sequentially evaporates. Thus, the organic semiconductor thin film having a continuous multi-layer (non-boundary multi-layer) structure can be manufactured where different organic compounds coexist between the organic layers. Especially, the composite solution including at least two organic EL materials are used, and the organic materials are controlled to deposit on the anode in the order of hole injecting-hole transporting-light emitting-electron transporting-electron injecting layers to form an organic EL device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/918,872 filed on Aug. 16, 2004, which is a continuation ofInternational Patent Application No. PCT/KR2003/000305 filed Feb. 13,2003, which claims priority of pending Korean Application No. 2002-8269,filed Feb. 15, 2002.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing an organicsemiconductor thin film, and an organic semiconductor device and anorganic electroluminescent (EL) device manufactured by the same, andmore particularly to, a method for manufacturing an organicsemiconductor thin film by a wet process, and an organic semiconductordevice, such as an organic EL device, manufactured by the same.

BACKGROUND OF THE INVENTION

Generally, organic semiconductor devices including organic diode devicesand organic transistor devices are based on the electricalsemi-conductivity that relates to the HOMO (Highest Occupied MolecularOrbital) energy level and the LUMO (Lowest Unoccupied Molecular Orbital)energy level of organic materials. Examples of the organic diode devicesinclude organic light emitting diodes and organic electroluminescent(EL) diodes, and examples of the organic transistor devices includeorganic FET (Field Effect Transistors), organic TFT (Thin FilmTransistors), organic SIT (Static Induction Transistors), organic topgate SIT, organic triodes, organic grid transistors, organic thyristors,and organic bipolar transistors. In these organic semiconductor devices,the electrical and optical properties of the devices are stronglydepending on the thin film structure of the organic layers formed on asubstrate. Thus, the development of the thin film having an efficientstructure is technically as important as the development of new organicmaterials. The present invention relates to a method for manufacturingan organic semiconductor thin film having a new structure with highefficiency, and a device including the organic semiconductor thin film.The present invention can be widely applied to the above-mentionedvarious organic semiconductor devices.

Hereinafter, the present invention will be described with reference tothe organic EL device, which has the simplest structure among theabove-mentioned devices. In the organic EL device, a thin film includingfluorescent organic compounds is positioned between electrodes, cathodeand anode. When a driving voltage is applied to the organic EL device,electrons and holes are injected into the LUMO and HOMO levels of thefluorescent organic compound of the thin film from the cathode andanode, respectively, and the injected electrons and holes are recombinedto produce excitons, which emit light (fluorescence or phosphorescence)through losing their activity. In the present invention, alight-emitting device represents an image display device using theorganic EL device. The light-emitting devices also include the followingmodules: a module formed by mounting a connector such as an anisotropicconductive film, FPC (Flexible Printed Circuit), TAB (tape automatedbonding) tape or TCP (Tape Carrier Package) to the EL device, a modulewhere a printed circuit board is installed at the end of the TAB tape orTCP, and a module formed by mounting an IC directly on the EL device bya method of COG (chip on glass). Recently, a method for producing anorganic or inorganic semiconductor device on a substrate has beenconsiderably developed, and an active matrix display device(light-emitting device) including the organic or inorganic semiconductordevice has been also being developed. In the present invention, thesemiconductor device also represents a single device or a plurality ofdevices, which have a switching function.

The organic EL device (also referred as ‘EL display device’) is a selflight-emitting device, and can be produced as a simple passive matrixlight-emitting device or an active matrix light-emitting device usingTFT. In the organic EL device, organic EL layers are positioned betweenelectrodes, as shown in FIG. 1 a. As shown in the figure, the organic ELlayers generally have a multi-layered structure, in which the boundaryor interface of each layer is clearly distinguished. The representativeexample of the multi-layer structure, suggested by Tang, et al.,includes a hole transporting layer 13, a light-emitting layer 14 and anelectron transporting layer 15 (Tang. C. et al. Appl. Phys. Lett. 1987,51, 913-915). This structure shows high light-emitting efficiency andthus the structure is adopted in almost all kinds of EL devices. Anotherexamples of the multi-layer structure include a structure having a holeinjecting layer 12, a hole transporting layer 13, a light-emitting layer14 and an electron transporting layer 15 which are sequentially formedon an anode 11 of a substrate 10, and a structure having a holeinjecting layer 12, a hole transporting layer 13, a light-emitting layer14, an electron transporting layer 15 and an electron injecting layer 16which are sequentially formed on an anode 11 of a substrate 10 (See FIG.1 a). The light-emitting layer 14 can be doped with fluorescent dopants.Besides the monomeric low molecular weight EL material, conjugatedpolymers such as poly(phenylvinylene) were introduced as the EL materialin 1990 by Burroughes et al. (Burroughes, J. H. Nature 1990. 347.539-541). Recently, stability, efficiency and durability of the polymerEL material have been remarkably improved. In this specification, alllayers sandwiched between the electrodes are called as the EL layers.Accordingly, the EL layer 20 includes the hole injecting layer 12, thehole transporting layer 13, the light-emitting layer 14, the electrontransporting layer 15 and the electron injecting layer 16. When avoltage is applied to the EL layer 20 from the electrodes 11, 17, theelectron-hole are recombined at the light-emitting layer 14 to inducethe light-emission. In this specification, the EL device also representsthe light-emitting device including the electrodes 11, 17 and the ELlayer 20. In order to prevent the EL device from being deteriorated dueto the exterior air, the substrate (EL panel) on which the EL device hasbeen formed is encapsulated with a sealing material (packaging), and isbonded to a cover member. Then, the connectors (FPC, TAB, etc.) aremounted for connecting the encapsulated EL device to an external drivingcircuit, which produces a passive or active matrix light-emittingdevice.

The EL layer 20 can be formed by various methods. Exemplary methodsinclude dry processes such as vacuum evaporation and sputtering, and wetprocesses such as spin coating, a cast method, an inkjet method, adipping method, and a printing method. Besides, roll coating, an LBmethod and ion plating method can also be used. In case of using a lowmolecular weight compound having a good thermal stability and capable ofbeing sublimated to form a thin film, the dry process such as vacuumevaporation is generally used to manufacture the multi-layer EL deviceshown in FIG. 1 a. However, the dry process requires a high vacuumenvironment, the manufacturing conditions should be controlledcarefully, and thus the process for fabricating EL devices is complex,resulting in the large manufacturing costs. On the contrary, the wetprocess, which uses a solution of the organic compounds dissolved in asolvent, and forms an organic layer of the dissolved compounds, has anadvantage in that the EL layer can be easily formed. Moreover, the wetprocess can be used not only for the low molecular weight compound butalso for the organic polymer materials. On the other hand, there aredisadvantages in the wet process, because of the solvent problem forforming multi-layer structure. Namely, different solvent must be used toform each different layer in forming the multi-EL layer. In this case,to form an upper organic layer on the lower organic layer, a solution,which does not dissolve the lower organic layer formed previously on thesubstrate, must be used to form the multi-EL layer. However, it isgenerally difficult to select appropriate set of the solvents to formthe multi-EL layers. If an improper solvent is used, the organic layerpreviously formed on the substrate 10 may be re-dissolved by the solventto form a new upper layer, and some of the organic materials of thelower or upper organic layers may irregularly diffuses into theneighboring layers. Thus, it is difficult to manufacture the multi-ELlayers 20 of FIG. 1 a by simply repeating the conventional wet process.Therefore, as shown in FIG. 1 b, the wet process is generally carriedout to form a single-EL layer 20 in which one or more compounds areuniformly dispersed in the single-EL layer 20. As a result, the ELdevice manufactured by the conventional wet process shows the lowlight-emitting efficiency and requires high driving voltage. In somecases, the multi-EL layer 20 can be formed by combination of the wetprocess and the dry process. However, the multi-EL layer 20 producedwith this method also has the low light-emitting efficiency and requireshigh driving voltage.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for solvingthe foregoing problems of an organic semiconductor thin film, such as anorganic EL thin film manufactured by the conventional wet process.Another object of the present invention is to provide an organicsemiconductor device, which can be easily manufactured and have improvedoperation reliability. Yet another object of the present invention is toprovide an organic EL device having improved quality of display images.Yet another object of the present invention is to provide an economicaland efficient EL display device produced with reduced costs.

In order to achieve these objects, it is provided a method formanufacturing an organic semiconductor thin film by a wet process, whichuses a composite solution prepared by dissolving at least two organicsemiconductor materials in a mixed solvent including at least twoorganic solvents having different volatility and having differentsolubilities of the organic semiconductor materials at room temperature,and an organic semiconductor device manufactured by the same. If theorganic semiconductor device is an organic EL device, the organicsemiconductor materials include organic compounds having electrical andoptical properties for injecting or transporting hole, light-emitting,and transporting and injecting electron et. al.

When the composite solution is used to form a device on a substrate by aprinting method, an inkjet method, spin coating or a dipping method, thesolvents of the composite solution are sequentially evaporated due tothe differences of volatility, and the organic materials having thelower solubility to the residual solvents are sequentially depositedonto the substrate due to the differences of solubility. Therefore, theorganic layers are sequentially deposited in the form of a continuousmulti-layer structure having indistinct boundaries. Here, each organiclayer may include a small amount of a different kind of organicmaterials of the neighboring organic layers, and the continuous boundaryregion between the neighboring organic layers includes at least twokinds of materials of the neighboring organic layers in mixed form.Namely, the organic semiconductor layers of the present invention isformed so that the concentrations of at least two organic semiconductormaterials (compounds) change with a gradient along with their depositiondirection at the interface. The thin film of the present invention formsa continuous multi-layer structure, which is different from the simpleconventional multi-layer thin film (FIG. 1 a) or uniformly distributedsingle-layer thin film (FIG. 1 b).

It is also provided a method for manufacturing an EL layer having acontinuous multi-layer structure by using a composite solution designedto sequentially deposit a hole injecting material, hole transportingmaterial, light-emitting material, electron transporting material, andelectron injecting materials on an anode. It is known that a single orsimply mixed organic solvent can be used in the wet process formanufacturing the EL thin film. For example, Korean Patent PublicationNos. 2001-0110183, 2001-0078227 and 2000-0062303 disclose that the ELlayer can be formed by dissolving EL material in a single or mixedsolvent. However, any of them does not suggest the method for formingthe continuous and non-boundary multi-layer structure which is providedby the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a structure diagram for illustrating the conventionalorganic EL device having a multi-layer structure;

FIG. 1 b is a structure diagram for illustrating the conventionalorganic EL device having a uniformly dispersed single-layer structure;

FIG. 2 a is a structure diagram for illustrating an organic EL devicehaving a continuous non-boundary multi-layer organic semiconductor thinfilm in accordance with the first embodiment of the present invention;

FIG. 2 b is a structure diagram for illustrating an organic EL devicehaving a continuous non-boundary multi-layer organic semiconductor thinfilm in accordance with the second embodiment of the present invention;

FIGS. 3 a and 3 b are graphs showing V-I and V-L characteristics of theEL device in accordance with the first embodiment of the presentinvention;

FIGS. 4 a and 4 b are graphs showing V-I and V-L characteristics of theEL device in accordance with the second embodiment of the presentinvention;

FIGS. 5 a and 5 b are graphs showing V-I and V-L characteristics of anEL device in accordance with the comparative example 1; and

FIGS. 6 a and 6 b are graphs showing V-I and V-L characteristics of anEL device in accordance with the comparative example 2.

DETAILED DESCRIPTION OF THE INVENTION

An organic semiconductor device and an organic EL device manufactured bya wet process in accordance with the present invention will now bedescribed in detail with reference to the accompanying drawings. In thefollowing description, same reference numerals are used for the sameelements even in different drawings.

In the EL device of the present invention, the continuous non-boundarymulti-layer structure of an organic semiconductor thin film includes atleast two layers. For example, it may have one of the followingstructures. Here, “˜” means the continuous non-boundary interface, while“/” means the distinct interface.

(1) anode/hole injecting, transporting layers˜light-emittinglayer/cathode

(2) anode/light-emitting layer˜electron transporting, injectinglayers/cathode

(3) anode/hole injecting, transporting layers˜light-emittinglayer˜electron transporting, injecting layers/cathode.

The organic semiconductor thin film suggested by the present inventioncan be forced to be formed by depositing compounds in the order of holeinjecting layer, hole transporting layer, light-emitting layer, electrontransporting layer and electron injecting layer on the anode. Thethickness of the organic semiconductor thin film ranges from 0.001 to 1μm, which is not intended to be limiting. FIG. 2 a shows therepresentative example of the organic EL device of the presentinvention. As shown in FIG. 2 a, the organic EL device includes anorganic semiconductor thin film, namely an EL layer 20 formed bysequentially depositing at least two organic semiconductor materials(compounds) between electrodes 11, 17. The organic semiconductormaterials form a hole injecting layer 12, a hole transporting layer 13,a light-emitting layer 14, an electron transporting layer 15 or anelectron injecting layer 16, and the concentration of the materialchanges with a gradient along with its deposition direction at theinterfaces. In addition, as shown in FIG. 2 b, a hole injecting layer12, a hole transporting layer 13, a light-emitting layer 14 and anelectron transporting layer 15 can be produced to a continuousnon-boundary multi-layer structure, and an electron injecting layer 16can be formed by a conventional wet process or a dry process such asvacuum evaporation on the non-boundary multi-layer.

In the present invention, the vacuum evaporator to form an organicmulti-layer is not necessary, thus the whole manufacturing system can besimplified and easily maintained. In addition, the present invention canbe applied to an active matrix EL device as well as a passive matrix ELdevice.

The organic EL device of the present invention can be manufactured bycoating a composite solution prepared by dissolving at least two organiccompounds in a mixed solvent including at least two organic solventshaving different volatility and having different solubilities of theorganic compounds, on the substrate where the electrode has been formed,and by evaporating the organic solvents from the coated compositesolution to sequentially deposit the organic compounds. The compositesolutions used to produce the organic EL device may include at least oneorganic light-emitting semiconductor compound emitting red, green orblue light. Preferably, the composite solution is optimized to enablethe devices to display wide ranges of colors (for example, 460, 520 and650 nm of narrow lines for B, G and R). The light-emitting materials ofthe organic EL device of the present invention are not limitative, andthus a variety of the conventional compounds for manufacturing anorganic EL device can be used in the present invention. Preferably, lowmolecular weight fluorescent materials or fluorescent polymer materialshaving the light-emitting property can be used, and the mixture of thelow molecular weight materials and the polymer materials can also beused.

Exemplary low molecular weight organic compounds used as the greenlight-emitting materials include Alq₃, BeBq₂(10-benzo[h]quinolinol-beryllium complex), and Almq(tris(4-methyl-8-quinolinolate)aluminum) which emit light in a greencolor range (550 nm). Typically, a few mol % of quinacridone, coumarin,C545T (Eastman Kodak Co.), or Ir-complex can be added (doped) to improvelight-emitting efficiency and durability. Also, exemplary dopingmaterials of a red light-emitting layer include Indigo, Nile Red, DCM(4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran),DCJTB (Eastman Kodak Co.), and Pt-complex. Exemplary blue light-emittingmaterials include metal complexes such as ZnPBO((Bis[2-(2-benzoxazolyl)phenolato]Zinc(II)) and Balq(Bis(2-methyl-8-quinolinolato) (para-phenyl-phenolato) aluminum), ornon-metal complexes such as styrylarylene derivatives, namely DPVBi(4,4′-bis(2,2′-biphenylvinyl)-1,1′-biphenyl), oxadiazole derivatives,bisstyryl anthracene derivatives, and bisstyryl arylene derivatives suchas BczVBi (4,4′-Bis((2-carbazole)vinylene)biphenyl). However, thelight-emitting materials of the organic EL device of the presentinvention are not specifically limited, and thus the aforementionedmaterials are not intended to be limiting. Exemplary polymer organiccompounds, used as the light-emitting materials of the organic ELdevice, include poly(p-phenylene), polyphenylene-vinylene, polyarylene,polyalkylthiophene and polyalkylfluorine. When the fluorescent polymermaterials are used for the light-emitting layer of the organic ELdevice, the fluorescent polymer materials can be a block, random orgraft copolymers, which also are not intended to limiting the presentinvention.

In the organic EL device of the present invention, exemplary holeinjecting and hole transporting materials include soluble phthalocyaninecompounds, aromatic diamine compounds such as TPD((N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine:triphenylamine derivative), MTDATA (4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine), quinacridone, bisstyryl anthracenederivatives, PVK (polyvinyl carbazole), porphyrinic compounds, α-NPD(N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine),polyaniline, and conductive polymers, which is not intended to belimiting. Exemplary electron injecting and electron transportingmaterials include Alq₃ which is an aluminum complex prepared bycoordinating three hydroxyquinolines on aluminum atoms, anddistyrylbiphenyl derivatives, which is not intended to be limiting.

In the organic EL device of the present invention, the light-emittinglayer and other organic layers can be formed to a thin film with anappropriate binder resin. If necessary, an appropriate dopant can beincluded in the layer. Exemplary binder resins includepolyvinylcarbazole (PVK) resins, polycarbonate resins, polyester resins,polyallylate resins, butyral resins, polyvinylacetal resins,diallyphthalate resins, acrylic resins, methacrylic resins, phenolresins, epoxy resins, silicone resins, polysulfone resins and urearesins. The resins can be used alone or as a copolymer including two ormore resins, which is not intended to be limiting.

Exemplary solvents for forming the mixed solvent of the presentinvention include methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butylalcohol, dimethylformamide, dimethylacetamide, ketone, acetone,diacetone alcohol, keto-alcohol, dioxane, ether, polyethylene glycol,polypropylene glycol, polyalkylene glycol, ethylene glycol, propyleneglycol, butylene glycol, triethylene glycol, hexylene glycol, diethyleneglycol, glycerol, ethyleneglycol monomethyl ether, diethyleneglycolmethylether, triethyleneglycol monomethylether, 2-pyrrolidon, toluene,xylene, chlorobenzene, dichlorobenzene, chloroform, dichloromethane,dichloroethane, gamma-butyl lactone, butyl cellosolve, cyclohexane, NMP(N-methyl-2-pyrrolidon), cyclohexanone, tetrahydrofurane (THF), carbontetrachloride, tetrachloroethane, octylbenzene, dodecylbenzene,quinoline, trichlorobenzene, nitrobenzaldehyde, nitrobenzene, carbondisulfide, 2-heptanone, benzene, terpineol, butylcarbitolacetate and ionexchange water (pure water). The above-mentioned solvents are typicalexamples of the solvents which can be used in the present invention, andthe present invention is not limited to the listed solvents. In thepresent invention, the selection of the at least two organic solventshaving different volatilities may depend on the property of the organiccompounds. Generally, organic solvents having boiling point differencemore than 3° C., preferably 5° C., more preferably 10° C. can be used.If the difference in volatilities of the organic solvents is too small,the evaporation of the organic solvents must be slowly carried out.Otherwise, the organic compounds may not be deposited sequentially. Inaddition, the organic solvents are selected to sequentially deposit theorganic compounds due to the organic compounds' solubility difference.If the solubilities of the organic compounds with respect to theselected solvents are the same, it is impossible to form the organicthin film having a concentration gradient according to the presentinvention.

The viscosity of the composite solution may affect the thickness of theEL layer, and the thickness of the EL layer is controlled to optimizethe emitting intensity. The viscosity of the composite solution can beadjusted by the selection of solvents, preferably less than 5000 cp. Thelower limit of the viscosity is not important in the present invention,but for example more than 100 cp, more preferably more than 1000 cp. Theconcentration of the EL materials in the solvents is determined so as tobe suitable for the wet process, preferably from 0.005 to 10 wt %, morepreferably 0.01 to 10 wt %. If the viscosity and concentration are awayfrom the above ranges, the film formation by the wet process may not beefficiently performed.

When the EL layer 20 is formed by using the composite solution accordingto the wet process such as the spin coating, cast method, inkjet method,dipping method and printing method, the EL layer 20 may be deteriorateddue to the moisture and oxygen of air. In order to remove moisture andoxygen, the EL layer 20 is preferably manufactured with a film formationdevice installed in a booth filled with low reactive gases, for examplerare gases or inert gases such as argon, helium and nitrogen.Thereafter, the solvents to form the EL layer 20 are sequentially andcompletely removed by thermal evaporation. Preferably, the solvents areevaporated at a temperature lower than a glass transition temperature ofthe EL materials. In addition, the EL layer 20 can be formed withpolymer precursors, and then the precursors can be transformed intopolymer EL materials by heating or UV curing.

Then, the cathode 17 (or anode 11) is formed on the EL layer 20 formedon the anode 11 (or cathode 17) of the substrate 10. In general, theanode 11 is preferably made of a material having a high work function.Exemplary anode 11 materials include silver, nickel, gold, platinum,palladium, selenium, rhenium, iridium, alloys thereof, tin oxide,indium-tin-oxide, and copper iodide. In addition, conductive polymerssuch as polyaniline, poly(3-methylthiophene), polyphenylene sulfide andpolypyrrole can be the materials for forming the anode. On the contrary,the cathode 17 is preferably made of a material having a low workfunction. Exemplary cathode materials include Al, Mg, Li, Cs, Ba, K, Beor Ca. Also, an electrode including MgAg (Mg:Ag=10:1) is preferablyused, and MgAgAl electrode, LiAl electrode and LiF/Al electrode can beused. Optionally, a protective electrode to protect the cathode fromexternal moisture can be formed, and the materials including Al or Agcan be used as the protective electrode.

Examples of the substrate 10 on which the EL layer 20 and the electrodes11, 17 are formed include a transparent substrates made of glass, quartzor polymer and inorganic semiconductor substrates made of silicon orgallium arsenide, which is not intended to be limiting the presentinvention. Finally, in order to protect the EL device from externaloxygen and moisture, the EL device is encapsulated with a sealing membersuch as glass, ceramic, plastic and metal under the inert gasatmosphere, or encapsulated with a thermosetting resin or ultravioletray curable resin. In addition, it is preferable to insert a hygroscopicmaterial in the encapsulated space, and the representative example ofthe hygroscopic material is barium oxide.

In this specification, the device having one pixel is mainly disclosedto illustrate the present invention. However, a plurality of pixelshaving the same structure can be aligned in a matrix type to form thedevice of the present invention, and the color EL display device canalso be manufactured according to the present invention. In addition,the present invention also can be applied to the color display devicesin which a white EL device and a color filter are combined, a blue orbluish green EL device and a fluorescent color covering material layerare combined, or a transparent electrode is used as a cathode and an ELdevice corresponding to RGB is respectively laminated. It is alsopossible to manufacture a black and white display device by forming awhite light-emitting layer on an EL layer. Example of the active matrixorganic EL display according to the present invention includes a thinfilm transistor switching device. However, the present invention is notlimited thereto, and other switching devices, such as two terminaldevices, for example, MIM can also be used. Moreover, passive driving,static driving and segment display driving can also be used in thepresent invention. In addition, a single or plurality of switchingdevices can be formed on one pixel.

The organic EL device of the present invention will be better understoodby referring to the following examples:

EXAMPLE 1

In this example, low molecular weight compound and polymer material wereused as the organic semiconductor EL materials. PVK(poly-N-vinylcarbazole, molecular weight: 150,000, T_(mp)=277° C.,T_(g)=156° C.) was used as the charge carrier binder resin, α-NPD (4,4bis[N-(1-napthyl-N-phenyl-amino)biphenyl]) was used as the holetransporting material, and green light emitting Alq₃(tris(8-quinolinolato)aluminum) was used as the light-emitting materialand the electron transporting material. 1:1 (weight ratio) mixture ofchloroform and dichloroethane (ClCH₂CH₂Cl) was used as the mixedsolvent. Here, the boiling points of chloroform and dichloroethane were62° C. and 82° C., respectively. Firstly, in order to evaluate thedepositing speed and depositing order of each organic material in themixed solvent, the EL materials were dissolved in the mixed solvent inthe amount of 0.05 wt %. The depositing order of the EL materialsaccording to the evaporation of the mixed solvent was evaluated. As theevaporation proceeded, chloroform was firstly evaporated, and α-NPD andPVK having lower solubilities in the dichloroethane were deposited toform a thin film. At the same time, a small amount of Alq₃ was alsoco-deposited. When chloroform was almost completely evaporated,deposition of α-NPD was ceased. Then, dichloroethane was evaporated, andAlq₃ dissolved in the solution (dichloroethane) was deposited with PVKto form another thin film on the lower α-NPD and PVK layer. On theboundary between the α-NPD and Alq₃ thin films, α-NPD and Alq₃ arecoexisted, and PVK was evenly dispersed on the whole thickness range ofthe thin films. Therefore, the mixed solution was useful for depositingthe continuous non-boundary α-NPD˜Alq₃ multi-layer ((1) anode/holeinjecting and transporting layers˜light-emitting layer/cathode), and PVKworked as the charge carrier binder resin.

The EL device according to an embodiment of the present invention wasformed as follows. An organic EL layer was made of an organicsemiconductor thin film formed by continuous depositions thereof.

(a) A glass substrate coated with Indium-Tin-Oxide (ITO) wasultrasonically washed in a commercially available cleaning agent, andthen washed with deionized water.

(b) The composite solution (chloroform:dichloroethane=1:1), in whichPVK, α-NPD and Alq₃ organic materials were dissolved, was filteredthrough 0.2 μm Teflon filter. Thereafter, the composite solution wasspin-coated on the ITO for 3 minutes under a spinning speed of 3000 rpm.

(c) The EL layer was thermally treated at 80° C. for 30 minutes tocompletely evaporate the solvents in the EL layer. The thickness of theproduced thin film was 500 to 700 Å.

(d) 2000 Å of Al:Li cathode was deposited on the EL layer with vacuumevaporation. The degree of vacuum was 5×10⁻⁶ torr, and the depositionspeed was 10 Å/second. Thereby, a circular shape organic EL devicehaving a diameter of 4 mm was obtained. In order to protect the ELdevice from the external environment, the EL device was encapsulated ina dry globe box.

The produced (ITO/α-NPD(PVK)˜Alq₃(PVK)/Al:Li) EL device had thelight-emitting initiation voltage (Vonset) of ˜13V, and the currentflowing through the EL device and the EL intensity at the voltage of 20Vwere 3.5 mA and ˜196 cd/m², respectively. The EL device emitted theuniform green light (540 nm). In addition, when the device was driven ata constant voltage of 20V, the organic EL display panel performed thestable light emission for a long time. The characteristics of the ELdevice are summarized in Table 1, and the voltage-current (V-I) andvoltage-EL intensity (V-L) properties of the EL device are shown inFIGS. 3 a and 3 b, respectively.

EXAMPLE 2

The same organic materials as the EL materials of Example 1 were used inthis Example. 1:1 (weight ratio) mixture of dichloroethane anddichloromethane (CH₂Cl₂) was used as the mixed solvent. Here, theboiling points of dichloroethane and dichloromethane were 82° C. and 40°C., respectively. The deposition speed and deposition order of eachorganic material from the mixed solvent were evaluated.

As the evaporation proceeded, dichloromethane was firstly evaporated,and α-NPD and PVK having lower solubilities in dichloroethane weredeposited to form a thin film. At the same time, a small amount of Alq₃was also deposited. When dichloromethane was almost completelyevaporated, deposition of α-NPD was ceased. Then, dichloroethane wasevaporated, and Alq₃ dissolved in the solution (dichloroethane) wasdeposited with PVK to form another thin film on the lower α-NPD and PVKlayer. On the boundary between the α-NPD and Alq₃ thin films, α-NPD andAlq₃ are coexisted, and PVK was evenly dispersed on the whole thicknessrange of the thin films. Therefore, the mixed solution was useful fordepositing the continuous non-boundary α-NPD-Alq₃ multi-layer, and PVKworked as the charge carrier binder resin.

The EL device according to an embodiment of the present invention wasformed according to the method described in Example 1 except for usingthe mixed solvent (dichloroethane:dichloromethane=1:1). The produced ELdevice had the light-emitting initiation voltage (Vonset) of ˜13V, andthe current flowing through the EL device and the EL intensity at thevoltage of 20V were 2.9 mA and ˜210 cd/m², respectively. The EL deviceemitted the uniform green light (540 nm). In addition, when the devicewas driven at a constant voltage of 20V, the organic EL display panelperformed the stable light emission for a long time. The characteristicsof the EL device are summarized in Table 1, and the voltage-current(V-I) and voltage-EL intensity (V-L) properties of the EL device areshown in FIGS. 4 a and 4 b, respectively.

COMPARATIVE EXAMPLE 1

The same organic materials as the EL materials of Example 1 were used inthis Comparative Example. 1:1 (weight ratio) mixture of chloroform andtoluene was used as the mixed solvent. Here, the boiling points ofchloroform and toluene were 62° C. and 110° C., respectively. Thedeposition speed and deposition order of each organic material from themixed solvent were evaluated. As the evaporation proceeded, chloroformwas firstly evaporated, and Alq₃, α-NPD and PVK having lowersolubilities to toluene were deposited to form a non-uniform thin film.When chloroform was completely evaporated, the depositions of Alq₃ andα-NPD were ceased. Then, toluene was evaporated, and PVK dissolved inthe solution was deposited to form a uniform thin film. Thus, the mixedsolvent was not useful for forming the continuous non-boundarymulti-layer.

The EL device was formed according to the method described in Example 1except for using the mixed solvent (chloroform:toluene=1:1). Theproduced EL device had the light-emitting initiation voltage (V_(onset))of ˜18V, and the current flowing through the EL device and the ELintensity at the voltage of 20V were less than 0.3 mA and ˜1 cd/m²respectively. The EL device emitted the non-uniform and unstable greenlight. The characteristics of the EL device are summarized in Table 1,and the voltage-current (V-I) and voltage-EL intensity (V-L) propertiesof the EL device are shown in FIGS. 5 a and 5 b, respectively.

COMPARATIVE EXAMPLE 2

The same organic materials as the EL materials of Example 1 were used inthis Comparative Example. The single solvent of chloroform was used asthe solvent. The deposition order of each organic material from thesolvent was evaluated. As the chloroform evaporated, Alq₃, α-NPD and PVKwere deposited to form a uniform thin film. Thus, the single solvent wasuseful for forming the single-layer where α-NPD and Alq₃ were uniformlydispersed, but was not useful for forming the continuous non-boundarymulti-layer.

The EL device was formed according to the method described in Example 1except for using the single solvent (chloroform). The produced organicthin film was a single layer in which the organic materials areuniformly dispersed. The produced EL device had the light-emittinginitiation voltage (V_(onset)) of ˜19V, and the current flowing throughthe EL device and the EL intensity at the voltage of 20V were less than0.5 mA and ˜1 cd/m² respectively. The EL device emitted the non-uniformand unstable green light. The characteristics of the EL device aresummarized in Table 1, and the voltage-current (V-I) and voltage-ELintensity (V-L) properties of the EL device are shown in FIGS. 6 a and 6b, respectively.

Therefore, the EL device having the organic semiconductor thin film withcontinuous non-boundary multi-layer structure of Examples 1 and 2 havemuch better operation efficiency than the EL device having single layerthin film of Comparative Examples 1 and 2. The properties of theproduced EL devices are summarized in Table 1. TABLE 1 Shape ofV_(onset) EL intensity Current at thin film (V) at 20 V 20 V (mA)Example 1 Non-boundary 13.0 195.6 3.5 multi-layer Example 2 Non-boundary13.0 209.5 2.9 multi-layer Comparative Uniformly 18.0 0.6 0.3 Example 1distributed multi layer Comparative Uniformly 19.0 0.8 0.5 Example 2distributed layer

As described, the organic semiconductor thin film having thenon-boundary multi-layer structure of the present invention is differentfrom the conventional uniformly distributed single-layer film ormulti-layer film, and can be easily manufactured. The organic EL devicehas the lower driving voltage and higher operation efficiency than thedevices produced with the single or simply mixed solvents. Thus, it ispossible to reduce the manufacturing cost of the efficientlight-emitting device and produce an electronic device having highquality of display images. The organic EL device of the presentinvention can be applied to various display devices, televisions,digital cameras, computers, notebook computers, mobile computers,portable image recording or displaying device, screens, bulletin boards,store signs, goggle type displays, car displays, video cameras, printerdisplays, remote control devices, phone displays, mobile phones, etc.

As shown in the above Examples, the organic semiconductor thin film ofthe present invention having the non-boundary multi-layer structure hasthe excellent “applied voltage”:“light emitting intensity” property, and“applied voltage”:“current” properties, which are similar to non-linearcurrent properties of a typical diode device. As a result, the organicsemiconductor thin film of the present invention can also be applied tovarious organic semiconductor devices such as organic diode devices.

While the present invention has been shown and described with referenceto certain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A composite solution for producing an organic thin film comprising: at least two organic compounds to form the organic thin film; and a mixed organic solvent including at least two organic solvents having different volatility and having different solubilities of the organic compounds.
 2. The composite solution of claim 1, wherein the at least two organic solvents are selected from the group consisting of methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tertbutyl alcohol, dimethylformamide, dimethylacetamide, ketone, acetone, diacetone alcohol, keto-alcohol, dioxane, ether, polyethylene glycol, polypropylene glycol, polyalkylene glycol, ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, hexylene glycol, diethylene glycol, glycerol, ethyleneglycol monomethyl ether, diethyleneglycol methylether, triethyleneglycol monomethylether, 2-pyrrolidon, toluene, xylene, chlorobenzene, dichlorobenzene, chloroform, dichloromethane, dichloroethane, gamma-butyl lactone, butyl cellosolve, cyclohexane, NMP (N-methyl-2-pyrrolidon), cyclohexanone, tetrahydrofurane (THF), carbon tetrachloride, tetrachloroethane, octylbenzene, dodecylbenzene, quinoline, trichlorobenzene, nitrohenzaldehyde, nitrobenzene, carbon disulfide, 2-heptanone, benzene, terpineol, butylcarbitolacetate and ion exchange water.
 3. The composite solution of claim 1, wherein a concentration of the organic compounds in the composite solution ranges from 0.005 to 10 wt %.
 4. The composite solution of claim 1, wherein a viscosity of the composite solution is less than 5000 cp. 